Alan W. Leslie, Armando Rosario-Lebron, Guihua Chen and Cerruti R. R. Hooks
Department of Entomology, College of Computer, Mathematical, and Natural Sciences
This extension article is meant to serve as a condensed write-up of a completed field study. Full-text of the published work can be viewed via open access at http://www.mdpi.com/2073-4395/8/6/87. Cover cropping has long been used as a method of reducing soil erosion, increasing soil quality, and suppressing weeds. However, impacts of cover crops in cropping systems differ and can be affected by timing and method of their termination. Field trials were conducted over two field seasons and at two sites in Maryland to examine how varying the date and method of terminating a barley (Hordeum vulgare) winter cover crop affects arthropods (insects and spiders) in succeeding no-till soybean (Glycine max) plantings. Experimental treatments included early-kill with pre- and post-emergent herbicides (EK), late-kill with pre- and post-emergent herbicides (LK), late-kill with a flail mower and pre-emergent herbicide (FM), and a fallow/bare-ground check with pre- and post-emergent herbicides (BG). Terminating barley late (i.e., just prior to soybean planting) resulted in significantly greater biomass accumulation in LK and FM than EK. However, method and timing of termination had no effect on communities of pest and beneficial arthropods in the soybean canopy. Results from this experiment suggest that terminating the cover crop early or late or using a mower or burn-down herbicide to kill the cover crop will result in similar species and number of arthropods within the soybean canopy.
Cover cropping can be a viable weed management tool in conservation agriculture systems. When cover crops are terminated in reduced- and no-till cropping systems, resulting residues that remain on the soil surface can help prevent weed establishment. Thus, it is well known that cover crop residue impacts weed populations. More specifically, some of these studies were designed to examine how method and timing of cover crop termination practices impact weed populations in grain crops. However, impacts of these practices on arthropod populations are rarely considered. Despite this, studies have shown that cover crops can impact arthropod numbers in succeeding agronomic crops. Some insect pests shown to be impacted by cover crop residue include the potato leafhopper (Empoasca fabae), bean leaf beetle (Cerotoma trifurcata), and Japanese beetle (Popillia japonica) in soybean as well as thrips in cotton (Gossypium hirsutum). In addition to insect pests, their natural enemies may be influenced by cover crop residue.
The overall goal of this study was to investigate how different cover crop termination practices impact populations of insect pests and their natural enemies within no-till soybean plantings. Specific objectives were to compare the influence of termination method (chemical versus mechanical) and timing (early versus late) on arthropod populations. Barley was chosen as the test cover crop partially because of its accessibility and popularity among producers.
Materials and Methods
Field experiments were conducted at the University of Maryland’s Central Maryland Research and Education Center at the Upper Marlboro and Beltsville farm sites in 2013 and 2014. Each field experiment consisted of four treatments, including three cover crop termination methods and a fallow/bare-ground control. The three cover crop treatments included: (1) early-kill (EK), in which the cover crop was sprayed with post- and pre-emergent herbicides in mid-April; (2) late-kill (LK), in which the cover crop was sprayed with post- and pre-emergent herbicides in late May; and (3) flail-mowed (FM), in which the cover crop was sprayed with a pre-emergent herbicide and mowed in late May. An early-kill, flail-mowed treatment was not included in the experiment because mowing typically does not kill cover crops at early stages of development and farmers do not use this method. The bare-ground treatment (BG) remained fallow after the previous crop was harvested and received the same post- and pre-emergent herbicide applications as LK.
The EK treatment was sprayed with a post- and pre-emergent herbicide mixture on 15 April at Beltsville and 16 April at Upper Marlboro in 2013 and on 18 April at both sites in 2014. The LK treatment was sprayed with a post- and pre-emergent herbicide mixture on the day soybeans were planted. The BG treatment received the same spray protocol as LK. On the day soybeans were planted, the FM treatment was sprayed with a pre-emergent herbicide and the cover crop was mowed. The soybean was planted on 21 May at Beltsville and 20 May at Upper Marlboro in 2013 and 27 May 2014 at both sites. Soybeans were planted in wide rows [76 cm (30 inch) inter-row spacing] at Beltsville and narrow rows [18 cm (7 inch) inter-row spacing] at Upper Marlboro. A late-season herbicide application was applied to all plots at the Beltsville location as a “rescue” herbicide treatment primarily for large crabgrass (Digitaria sanguinalis).
Data collection. Data on vegetative (cover crop and weed) biomass, abundance of weeds and arthropods, soil moisture as well as yield were collected during this investigation. Data on weed population, soil moisture and yield will be presented in a future edition of Agronomy News. To quantify cumulative barley and weed biomass production, cover crop and weed biomass were measured in each plot just prior to their termination. Arthropod abundances were monitored with the use of a sweep net from the R1 through R5 soybean growth stages. Arthropods collected were divided into i) natural enemies (predators – arthropods that prey on herbivores & parasitoids – insects, especially wasps, that complete their development within the body of another insect eventually killing it) and ii) herbivores (insects that feed on plants). Arthropods were separated further according to seven feeding habits (guilds). The seven feeding habits that we used included 1) chewing predators, 2) sucking predators, 3) parasitoids, 4) plant-sucking herbivores, 5) pod feeders, 6) foliage feeders and 7) spiders. Though they are predators, spiders were placed into a separate predatory feeding guild.
Vegetative biomass. At each farm site, flail-mowed (FM) and late-killed (LK) treatments had higher plant biomass than early-killed (EK) or bare-ground (BG) treatments (Table 1). Total barley biomass in LK and FM treatments were more than two times greater at Beltsville than Upper Marlboro. No differences were detected in plant biomass between BG and EK treatments within each site, but there was greater weed biomass in the BG treatment at Beltsville than Upper Marlboro (Table 1).
Table 1. Cover crop and weed dry biomass just prior to their termination.
|Site||Treatment||Mass ± SEM (kg ha−1)|
|Upper Marlboro||Early Kill||85.8||±||21.2||d|
1Different letters indicate that means are significantly different.
Arthropod Counts. In total, 54 families of arthropods were collected from sweep samples which included a total of 11,344 specimens (Table 2). Approximately 98% of arthropods collected could be assigned to one of the seven feeding guilds used. Three feeding guilds, which included plant-sucking herbivores (25%), foliage-feeding herbivores (24%), and sucking predators (21%), accounted for 70% of the entire arthropod community sampled.
The abundance of arthropods from each feeding guild was similar among treatments. However, there was a significant effect of soybean development stage on all feeding guilds. In general, parasitoid, chewing predator, and sucking predator guilds reached greatest abundance later in the season (R4 or R5 stage). In contrast, numbers of foliage feeding and plant sucking herbivores peaked earlier in the growing season at the R2 or R3 stage (Table 3). Sucking predators and spiders were found in greater numbers in Beltsville than Upper Marlboro across all soybean growth stages.
Table 2. Arthropod feeding guilds, families and their abundances. Numbers represent total abundance across all sample dates.
1Unspecified taxa were not identified to the family level.
Table 3. Means (± standard errors) of feeding guilds within farm site and soybean development stage.
|Abundance per 10 Sweeps|
1BV = Beltsville, UM = Upper Marlboro
2Different letters within individual rows represent significant differences between growth stages.
The objective of this study was to quantify the impact of cover crop termination method and timing on arthropods within soybean foliage. Cover crop termination practices are known to impact arthropods via resulting residues that remain on the soil surface. Thus, it was believed that different cover crop termination methods examined during this study would influence arthropod abundances differently. As expected, delaying the cover crop termination date resulted in significantly greater biomass of residue in late-kill (LK) and flail-mowed (FM) than in early-kill (EK) treatments. Averaged across years, delaying cover crop termination in FM and LK increased barley biomass relative to EK by 2007.5 kg ha−1 (1791 lbs/acre) at Beltsville and 716.8 kg ha−1 (639.5 lbs/acre) at Upper Marlboro. However, arthropod populations within the soybean foliage responded similarly to treatments regardless of plant biomass differences. Instead, arthropod abundances changed according to soybean growth stage. Chemical (LK) and mechanical (FM) termination tactics also had similar effects on arthropod abundances. This suggests that whether cover crops are killed early or late, or chemically or mechanically by mowing, the resulting arthropod community will be similarly impacted. The EK “early” (early April) and LK “late” (at soybean planting from mid to late May) treatments represent some of the most widely used practices for cover crop termination by Mid-Atlantic soybean producers. The results of our study suggest that these two practices are likely to result in similar species and number of foliar arthropods throughout the different soybean growth stages.
Overall, our results indicate that cover crop termination methods that result in greater cover crop biomass will have no effect on insects and spiders within the soybean foliage. However, if delaying cover crop termination results in greater weed suppression without impacting soybean productivity, this practice should nevertheless make soybean systems more resilient to pest pressure and acceptable by soybean producers.
We thank crews at the Upper Marlboro and Beltsville Research and Education Centers for logistics in establishing and completing field trials. This work or publication was supported by Hatch Project No. MD-ENTO-9107/project accession no. 227029 and the Crop Protection and Pest Management (CPPM), Extension Implementation Program (EIP) award number 2017-70006-27171 from the USDA National Institute of Food and Agriculture, and funding from the Maryland Soybean Board.